The Largest T. Rex Could Have Been 70 Percent Heavier Than Fossils Suggest

Around 66 million years ago, the iconic and terrifying Tyrannosaurus rex roamed the planet. Though scientists have only been able to get their hands on a small number of fossil skeletons of the dinosaur, they know it was one of the largest carnivores to ever walk the Earth.

Now, two researchers have used computer modeling to reveal that the biggest T. rex ever might have been 70 percent heavier and 25 percent longer than the largest T. rex skeleton known today. Their study was published last week in the journal Ecology and Evolution.

“Our study suggests that, for big fossil animals like T. rex, we really have no idea from the fossil record about the absolute sizes they might have reached,” study co-author Jordan Mallon, a paleobiologist at the Canadian Museum of Nature, says in a statement.

That’s because there are so few known T. rex skeletons—only about 30 have ever been found. So, it is statistically unlikely that these specimens would happen to represent the largest individuals of the species.

However, “some isolated bones and pieces certainly hint at still larger individuals than for which we currently have skeletons,” co-author David Hone, a paleontologist at Queen Mary University of London, adds in the statement.

To estimate just how big T. rex could have gotten, Mallon and Hone turned to computer modeling. They factored in attributes including the species’ growth rate, lifespan, population size, body size variation and the incompleteness of the fossil record.

While most of these details are well-understood for T. rex, the team needed to extrapolate body-size variance. They used two different methods—one that took into account potential sexual dimorphism, or a difference in size or appearance between males and females, and one that assumed sex did not affect a T. rex’s size. To build the model with dimorphism, the team followed the size differences of alligators, the T. rex’s modern-day kin.

“If T. rex was dimorphic, we estimate that it would have weighed up to 53,000 pounds (24,000 kilograms), but we rejected that model, because if it were true, we would have found even larger individuals by now,” Mallon tells Live Science’s Jennifer Nalewicki.

The current heavyweight champion represented by the known T. rex fossils is a specimen called Scotty, which weighed an estimated 19,555 pounds and grew 39 feet in length. In 2019, Steve Brusatte, a paleontologist at the University of Edinburgh in Scotland, told New Scientist’s Colin Barras that Scotty was “probably our best look yet at what one of the largest, oldest, most fully grown adults would have looked like.”

But Scotty now seems trivial next to Mallon and Hone’s final upper-bound estimate for the T. rex, which comes from their second model: 33,000 pounds and over 49 feet long. That’s heavier than the average school bus, which clocks in at around 24,000 pounds.

“This reminds us that what we know about dinosaurs isn’t much at all, since the sample sizes are so small,” Thomas Carr, a vertebrate paleontologist at Carthage College who wasn’t involved in the new research, tells Live Science. “To imagine a T. rex of that magnitude is extraordinary, and I think an animal of that size is within reach statistically.”

Scotty and other large T. rex fossils probably belonged to individuals that were in the top 1 percent biggest specimens of the population. To find fossils in the top 0.01 percent range would take paleontologists 1,000 more years of work, the team estimates, assuming excavations continue at the current rate.

“It’s important to stress that this isn’t really about T. rex, which is the basis of our study, but this issue would apply to all dinosaurs, and lots of other fossil species,” Hone says in the statement. “Arguing about ‘which is the biggest?’ based on a handful of skeletons really isn’t very meaningful.”

At the end of the day, the paleontologists stress that their results are “simply a thought experiment with some numbers behind it” for now, Mallon tells Live Science. Until paleontologists discover physical proof of such a gargantuan creature, all they can do is speculate.

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.